1. Field of the Invention
The present invention relates to a method for producing a mold for a zonal optical element, which is based on Japanese Patent Application No. 2004-21439 filed on Jan. 29, 2004, disclosure of which is incorporated herein by reference.
2. Description of Related Arts
Optical elements comprising a spherical surface having an optical axis at the center thereof and a plurality of projecting minute zonal portions formed on the circumference of the spherical surface, such as Fresnel lens and blaze type diffraction element, have been produced by utilizing a mold having a concave portion corresponding to the spherical surface and grooves for zones corresponding to the plurality of projecting minute zonal portions (hereinafter referred to as “zonal grooves) as a rule. In such a production process, it is general to cave the minute, zonal grooves through a tip as a rule. In this process, since the edge shape of the tip is copied onto the mold, it has been desired to point the edge shape of the tip (to sharpen the edge of the tip). However, only when the edge of the tip is sharpened, there remains a problem in terms of changing the surface coarseness of the processed surface for the worse. In such a situation, for example, two types of techniques have hitherto be known as methods for producing a mold for a zonal optical element.
Referring to FIG. 4, a first technique is mentioned, which is a method in which the an edge of tip 21 serving as a cutting tool is formed into a curve having a minute radius (minute round shape), to thereby form minute, zonal grooves onto the mold (for example, see Japanese Patent Laid-Open Publication 2003-62707) Specifically, as shown in FIG. 5, while a raked surface 21a of the tip 21 being at a right angle relative to a rotational direction of a column-shaped work W, the tip 21 allows for continuously moving along the direction of the diameter of the column-shaped work W and for moving it in the shaft direction of the work W as occasion demands, whereby the edge of the tip 21 allows for following up a waveform processing surface PF having been coarsely processed to form minutes, zonal grooves 7. This technique makes it possible to enhance the surface coarseness and to form a bottom surface of each zonal groove 7 in much smaller round shape in comparison with the case where the edge of the tip 21 is sharpened.
As a second method, a method for producing an optical element can be mentioned, in which a lithographic technique utilizing many masks to fake the slope of each zonal groove in a state of steps, whereby an optical element itself is produced (for example, see Japanese Patent Laid-Open Publication No. 9-230121). However, the production of the optical element utilizing such a lithographic technique is not suitable for mass production due to long processing period or such. Consequently, the optical element produced by the lithographic technique is used as a model, and the resulting model has often been used to make a reversal process for producing a mold for an optical element through electro-forming and other forming.
However, although the first technique can slightly enhance the surface coarseness in comparison with the processing where the edge of the tip is sharpened, the surface coarseness is not considered to be desirable comparing with the case of caving the processing surface by a generally used tip. In order to enhance the surface coarseness at fullest extent, feeding speed is extremely decreased to make a process in some cases. However, this leads to increasing of processing time, as well as increasing a possibility to changing the shape for the worse due to decreasing the temperature stability in processing environment. Furthermore, the tip is heavily worn since the edge of the tip is sharpened comparing with the generally used tip.
In the reversal process in the second technique, there remains problems in terms of technical difficulties for obtaining sufficient reversibility, and for peeling element without deformation. Moreover, when the slope is faked by steps, at least eight steps are required for the formation of the slope. This poses problems associated with the complexity of aligning the mask per every steps and the like.
An object of the present invention is, therefore, to provide a method for producing a mold for a zonal optical element which can enhance the surface coarseness without taking a longer period of time.